Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic disease that affects children in the first decade of life and causes a remarkable phenotype resembling many aspects of aging. Affected children develop an extremely aged appearance, a lack of subcutaneous fat, growth retardation and severe atherosclerosis. Affected children die of premature atherosclerosis at an average age of 13 years. Progeria is a disease in which some, but not all, of its manifestations (in vivo and in vitro) represent a model of accelerated aging (reviewed in Sweeney & Weiss, Gerontology 38:139-52, 1992). Clinical features common to progeria and normal aging include alopecia (although the pattern of hair loss differs), sclerodermatosis, atherosclerosis, lipofuscin deposition, nail dystrophy, hypermelanosis, decreased adipose tissue, and osteoporosis. Clinical differences include sequelae of maldevelopment in progeria, with coxa valga, distal bone resorption, delayed dentition, facial disproportion, failure to thrive, and short stature. Features of aging that are absent in progeria include neurosensory decline such as Alzheimer's disease, dementia, hearing loss, and presbyopia.
Recently, the gene responsible for HGPS was identified, and HGPS joined a group of syndromes—the laminopathies—all of which have an underlying defect in the lamin A/C gene (LMNA) (Eriksson et al., Nature 423:293-298, 2003). LMNA codes for the lamin A and lamin C isoforms, which differ due to alternate splicing, as well as the Δ10 isoform found in sperm. The lamins are a component of the nuclear lamina, a fibrous matrix located at the interior of the nuclear membrane, responsible for nuclear integrity and organization. In addition, lamins are also present in the nucleoplasm and may be involved in more complex spatial organization of the nucleus. They play a role in a wide array of nuclear processes, including transcription, replication, chromatin organization, nuclear shape, cell division, and cell cycle functions (Gruenbaum et al., J Struct Biol 129:313-23, 2000; Gruenbaum et al., Nat. Rev. Mol. Cell Biol. 6:21-31, 2005). The LMNA gene is primarily expressed in differentiated tissues in the fetus and adult and may be important in maintaining the differentiated state (Rober et al., Development 105:365-378, 1989). In fact, lamin A expression is down-regulated in many tumors, perhaps as part of the loss of differentiation seen in those tumors (Muller et al., Leukemia 8:940-945, 1994).
The pre-lamin A protein contains a CAAX box (SEQ ID NO: 31) at the carboxy terminus, which is an invariant cysteine followed by two aliphatic amino acids with the X denoting the terminal amino acid. The CAAX box signals for isoprenylation, the addition of a 15-carbon farnesyl isoprenoid lipid group to the cysteine by the enzyme farnesyltransferase (FTase) or a 20-carbon geranylgeranyl isoprenoid lipid by geranylgeranyltransferase I (GGTaseI) (Beck et al., J. Cell Biol. 110:1489-1499, 1990). The final amino acid defines the specificity for the addition of the isoprenyl group with methionine, serine, glutamine, or alanine signaling farnesylation and leucine signaling the addition of a 20-carbon geranylgeranyl isoprenoid group catalyzed by the structurally related enzyme GGTase I (Moores et al., J Biol Chem 266:14603-14610, 1991; Cox & Der, Curr. Opin. Pharmacol. 2:388-393, 2002). The native lamin A CAAX box (SEQ ID NO: 31) consists of CSIM (cysteine, serine, isoleucine, methionine) (SEQ ID NO: 32). Farnesylation, together with subsequent CAAX-signaled modifications, promote prelamin A association with the nuclear membrane (Hennekes & Nigg, J. Cell Sci. 107:1019-1029, 1994). Farnesylation is a permanent modification; once a farnesyl group is added to a protein, it remains attached to that residue for the life of the protein. Following farnesylation, the terminal three AAX amino acids are removed, and the C-terminal isoprenylated cysteine undergoes methyl esterification (Hennekes & Nigg, J. Cell Sci. 107:1019-1029, 1994). While both B-type lamins and lamin A are farnesylated and carboxymethylated, unique to lamin A is a second cleavage that occurs inside the nucleus causing the removal of an additional 15 C-terminal amino acids from the mature protein, including the farnesylated cysteine. Because farnesylation is a permanent posttranslational modification, proteolytic cleavage of the farnesylated cysteine is necessary for full processing of the prelamin A protein to mature lamin A, and for its correct subcellular localization and function. Thus, this final cleavage step and the resulting loss of the farnesyl anchor presumably releases prelamin A from the nuclear membrane and allows it to be inserted into the nuclear lamina. In HGPS, although preprogerin can be farnesylated, its internal deletion of amino acids 606-656 removes the endoprotease recognition site necessary for executing the final cleavage step. This final cleavage step appears to be important for normal function as mutations in ZMPSTE24 cause a severe form of mandibuloacral dysplasia (MADB), one of the laminopathies which is phenotypically similar to HGPS (Agarwal et al., Hum. Mol. Genet. 12:1995-2001, 2003). ZMPSTE24 is the human homolog of yeast STE 24 and is responsible for the final cleavage of lamin A that removes the 15 terminal amino acids (Pendas et al., Nat. Genet. 31:94-99, 2002).
Nearly all HGPS patients have the same silent mutation (G608G) creating an abnormal splice donor site in exon 11 of the LMNA gene (Eriksson et al., Nature 423:293-298, 2003), which causes a 150 base pair mRNA deletion in the lamin A transcript. The result of the mis-splicing is a protein missing 50 amino acids near the C-terminus (henceforth called “preprogerin” prior to posttranslational processing and “progerin” after post-translational processing). The deleted region includes the protein cleavage site that normally removes the C-terminal 15 amino acids, including the farnesylated cysteine. The deleted region also contains two potential cyclin-dependent kinase target serines (652 and 657) that may be involved in dissociation and reassociation of the nuclear membrane at each cell division (Sinensky et al., J Cell Sci 107 (Pt 1):61-7, 1994; Kilic et al., J Biol Chem 272(8):5298-304, 1997) and it may affect molecular solubility (Hennekes & Nigg, J Cell Sci. 107:1019-29, 1994).